Apply de Moivre's theorem for integer powers (#657)

Apply de Moivre's theorem for integer powers (#657)

[r(cos⁡θ+isin⁡θ)]n=rn(cos⁡nθ+isin⁡nθ)[r(\cos\theta + i\sin\theta)]^n = r^n(\cos n\theta + i\sin n\theta).